Bending fixtures have been used for many years in the fabrication of glass products from flat glass sheets and numerous types of bending fixtures are well known in the glass industry. Typically, a bending fixture provides a narrow support surface forming a ring or the like in a generally horizontal plane. A flat glass sheet is laid upon the support surface, the support surface contacting the bottom surface of the glass sheet at or near the outer periphery, usually about 5 to 15 mm from the edge of the glass. Therefore, the main area of the glass sheet is usually untouched during the bending process. For this reason, high quality surfaces can be produced. Automotive windshields are usually fabricated using bending fixtures, because of the industry's high quality requirements for windshields. In contrast, side and back windows often are formed using press forming equipment, although bending fixtures also can be used.
In a traditional method of gravity bending thin, flat sheets of glass into curved shapes such as automobile windshields, cold, pre-cut single or multiple glass sheets are placed onto the rigid, pre-shaped, peripheral metal support surface of a bending fixture. Prior to bending, the glass typically is supported only at a few contact points. The glass is heated, usually by exposure to elevated temperatures in a lehr, which softens the glass allowing gravity to sag or slump the glass into conformance with the peripheral support surface. Substantially the entire support surface generally will then be in contact with the periphery of the glass. The occasional slight surface markings along the periphery may subsequently be concealed by the glass framing means or the markings may be tolerated as inconsequential because they are located at the very edge of the vision area.
Bending fixtures, also known in the industry as bending irons and skeleton molds, are constructed according to various designs and may be comprised of any number of materials. Exemplary bending fixtures are disclosed in U.S. Pat. No. 3,068,672 to Black and in U.S. Pat. No. 3,161,493 to Golightly and in U.S. Pat. No. 3,356,480 to Golightly. Although the bending fixtures currently used in industry are not uniform in design, there are several common design features. For all but modest or shallow bends, the peripheral support surface is often divided into a relatively flat central portion and two, usually symmetrical, more deeply bent "wings." The wing portions of the bending fixture frequently are connected by hinges to the central portion to complete the continuous, peripheral support. The hinged wing portions are provided in the belief that unless the final shape of the support frame approximates a catenary curve, gravity alone will not force the hot glass into the desired shape or form. Specifically, the hinged wings are used to apply bending force to accelerate the bending of the glass sheet along the sharpest bend lines.
In some bending fixture designs, the "wing bending effort" is reinforced or magnified by the addition of fixed or adjustable-length counterweights. The counterweights help rotate the wings upwardly about the hinge axis (relative to the center portion of the bending fixture) into the closed position during the bending process. In other designs the wing bending effort is increased by the addition of various systems of suspension links and hinges to hold the wings up while the central area sags, such that the weight of the central glass portion provides leverage or motivating force for the wing bending action. In the open position, in which the bending fixture would support a flat glass sheet, the wings generally are in a horizontal plane with the central portion of the fixture. In this position the flat glass sheet typically is supported at the four points where the support surface is hinged, that is, at the four points where the central portion of the bending fixture is hinged to the two wings. The glass would also be supported at points laterally outward of the hinge points, specifically, by outer points on the wings. The glass sheet is unsupported, however, between the hinge points, that is, across the large central portion.
In most currently known bending fixtures used to produce, for example, symmetrical windshields, the number of hinges or pivots employed for the support surface and wing bending function varies from as few as four to as many as twelve. When in use, the bending fixtures are continuously cycled from temperatures near room temperature to temperatures which may reach 650 degrees Celsius. Because of this cycling, most fixture hinges are difficult to lubricate and maintain. It would be advantageous, therefore, to reduce the number of hinges, etc. used in a bending fixture.
Most support frames locate the peripheral support "parting lines," that is, the hinge axes extending between the wings and the central support rails, directly at or in the vicinity of the tightest curves, that is, along the bends having the shortest bend radii. The assumption is that the lever or fulcrum action of the hinged wing is of most benefit in advancing the bending of the glass along the axis of the tightest bend. In practice, this works fairly well if the glass to be shaped is relatively thick, its vertical height is not too large, and the installation angle (for example, the so called "lay-down angle" of a windshield in a car) is not too severe. Even then, optical distortion bands along the wing hinge axes are common and may be objectionable, particularly in windshields with deep bends. Additionally, because during the bending phase the "hinge junctions" of the curved support surface (between the wings and the central portion) represent "high points", that is points on which large portions of the total glass weight rest, troublesome "hinge kinks" frequently develop at these points. In their mild form, hinge kinks cause localized distortions in "reflected appearance" or "showroom beauty". In their more severe form, they are a frequent cause of "edge penetration" and may ultimately cause the finished product to be scraped.
With new automobile designs, windshield bending requirements have changed. Laminated windshields frequently now are made from thinner glass, often as thin as 1.8 mm, in contrast to 3.2 mm thick glass in earlier bent windshields. Also, windshield installation angles have increased to as much as 62 degrees or more from the vertical. Therefore, new windshields have a greater surface area and reduced width to height aspect ratios. The increased angle also results in a longer light path within the windshield and the refractive power of any disturbance or distortion in the glass is correspondingly greater.
Due to the increased width to height aspect ratios, thin cold glass now must span a larger top-to-bottom distance when loaded onto a bending fixture. The distances between the "cold" support points for large volume windshield designs have increased by as much as a factor of two in recent years. As noted above, a flat glass sheet positioned on a typical bending fixture in the open position is unsupported across the large central portion of the bending fixture between the hinge points at the wings. This causes much larger beam deflections and, so, increases the tensile stresses within the glass. This, in turn, increases production losses due to breakage of the glass prior to heating. It would be advantageous, therefore, to reduce the distance spanned by a flat sheet on a bending fixture.
Certain windshield designs, particularly in newer model automobiles, demand relatively tight wing bend radii, close to the short edge near the "A pillar". These bends prove to be difficult to produce in some cases using known bending fixture designs. Specifically, it is found that the wing "lever arm" is often too short to effectively bend the glass. This results in the glass having a tendency to "bridge" a part of the deep curve during the bending process and, thus, fail to achieve the required form or shape. It would be advantageous, therefore, to employ a bending fixture having a support surface which provides good conformance of the glass sheet to the desired shape during bending.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following disclosure and detailed description of certain preferred embodiments of the invention, when taken in conjunction with the accompanying drawings.